DK149728B - REGISTRATION CARRIER WITH AN OPTICALLY READABLE PHASE STRUCTURE AND APPARATUS FOR READING SUCH A CARRIER - Google Patents

Description

in 149728

BACKGROUND OF THE INVENTION 1. Field of the Invention The invention relates to a record carrier which contains information in an optically readable structure comprising trace-spaced regions alternating with intermediate regions, the regions having a different influence on a reading beam than the intermediate regions and strips located between the information tracks. The invention also relates to an apparatus for reading such a record carrier.

In the case of a circular disc-shaped record carrier, the information track may contain several concentric tracks or several nearly concentric tracks connected together so that they together form a helical track.

One such registration carrier is, inter alia, known from 15 "Philips Technical Review" 33, No. 7, pages 178-190. On this record carrier, a television program is optionally color coded in the spatial frequency of the areas and in the lengths of the areas. The width of the information tracks and thus the width of the areas is e.g. 0.5 µm, the trace period in the radial direction is e.g. 1, 7 µm and the average length of the areas e.g. 0,5ym. In an annular area with an inner radius of 6.5 cm and an outer radius of 14.5 cm it is possible to register a television program of approx. 30 minutes.

25 For certain programs, e.g. feature films, it is desirable with a longer playing time. A longer playing time could be achieved by placing the information tracks closer together.

When reading the record carrier, care must be taken to ensure that the center of the reading patch formed on the information structure coincides with the center of the track portion being read, otherwise the modulation depth of the read signal will be small and cross talk may occur between the tracks. Therefore, during the reading, a radial error signal is generated which gives an indication of the position of the reading spot relative to the center of the track portion being read. In a servo system, the radial position of the reading patch is easily corrected by error signal. For generating the radial error signal, the lattice-shaped structure of adjacent information tracks in the radial direction is used. The optical read system by which the information structure is read is adapted to the read information structure.

This means that the wavelength of the reading beam and the numerical aperture of the reading lens are selected in such a way that the areas of greatest spatial frequency, which are the areas of the inner track in the case of a 10 round disc-shaped record carrier, can still be read with satisfactory discrimination. For a given reading system, a compromise must be made with respect to the radial spatial frequency. The radial spatial frequency is chosen such that a certain playing time is obtained while the crossover between the tracks remains within a certain limit and the radial error signal is still sufficiently large. The radial trace period of 1.7 µm listed above applies to a wavelength of X = 0.63 µm and a numerical aperture N.A. = 0.45. If the radial spatial frequency was increased e.g. with a factor of 2 for doubling the playing time, this spatial frequency would be located near the boundary frequency of the optical system and radial position errors for the reading spot could then or hardly be detected.

25 In addition, the reading patch is larger than the width of the information traces. If the radial period of the information track was reduced, a significant portion of the reading radiation would hit neighboring tracks of the read track.

This would give rise to significant cross talk between the information tracks even if the reading spot was properly centered relative to the track read.

It is an object of the present invention to increase the information density of a record carrier without the disadvantages mentioned. The invention 35 is realized in the record carrier and the apparatus for reading it.

The recording carrier according to the invention is characterized in that adjacent formation grooves differ from each other in that they contain areas with a first phase depth and areas with a second phase depth, respectively.

The information is then contained in two structures 5 with different phase depth. The phase depth is defined as the phase difference between a zero-order and higher-order beams in which the reading beam is divided by the information structure. Each phase depth then corresponds to a specific arrangement of the detection system by which the structure in question can be read in an optimal way. A deep phase structure is read optimally by detecting the total intensity over the pupil of the reading lens, while a less deep phase structure is read optimally by determining the difference between the intensities in two tangentially different pupil halves. Using two different phase depths and different detector arrangements, the radial period of the information track can be reduced by e.g. a factor of 2, despite the fact that the information traces can still be read with 20 satisfactory discrimination.

If only the phase differences between the zero-order and higher-order sub-beams were to be taken into account, a phase depth of ir rad for the deep grooves and a phase depth of π rad for the low 25 grooves could be chosen. However, for a phase depth of π rad, the radiant energy in the higher order sub-rays is very small, so that the detector signals would also be small.

Therefore, in practice, a phase depth that is slightly larger than π rad for the low traces is chosen.

30 ^

Therefore, a preferred embodiment of a recording carrier according to the invention is characterized in that the first phase depth is approximately one row and the second phase depth approximately three to one row.

It is also possible to assign a phase depth, namely a phase depth of 4 169728 ττ rad, to an information structure containing radiation absorbing regions and radiation transmitting or radiation reflecting intermediate regions. Such an information structure is referred to as an amplitude structure. One of the two tracks of the recording carrier according to the invention may contain such an amplitude structure.

Preferably, the grooves with large phase depth contain and the grooves with small phase depth recesses or projections. The advantage of a record carrier with recesses or elevations is that it can be prepared quickly in large quantities using known pressing technique.

In the case of an information structure containing recesses or projections, the phase depth defined above as the phase difference between the zero-order and the higher-order sub-beams is related to a geometric phase depth.

The geometric phase depth φ for a reflective information structure is given by: φ = 2.2 it ^ rad, where d is the geometric depth of the indentations and λ the wavelength of the reading beam. For a radiation transmitting information structure, φ = 2 in ^ rad.

Compared to a radiation transmitting information structure, a reflective information structure has the advantage that the elements of the optical reading apparatus during the reading are all located on one side 25 of the recording carrier and partially traversed twice by the reading beam.

According to the invention, a record carrier having a reflective information structure may be further characterized in that the regions of the first phase depth are made up of depressions having the geometric depth of approximately 1/4 λ, and the regions of the second phase depth are made up of depressions with a geometric depth. of approximately 1/8 λ, with λ being the wavelength of the reading beam.

When reading a record carrier according to the invention, the signal from one detector arrangement and the signal from the other detector arrangement must be alternately transmitted to an electronic circuit in which the signals are further processed. The signal read »149728 5 is finally displayed e.g. by means of a television set, or made audible. The modulation transfer functions of the read systems with the different detector arrangements vary slightly. If the information is stored in digital form, the variations in the transfer functions will not be noticed in the signal which is eventually supplied to the reader. If the information is otherwise recorded, e.g. in the form of a frequency modulated signal, the variation between the modulation-transfer functions may become noticeable. For example, one transfer function will. give rise to other shades of gray or a different color saturation in the television signal than the other transmission function. In the case of an audible signal, the variation between the transmission functions may be audible as an undesirable frequency.

If a television program has been registered on the registration carrier, eg. contains one television picture per rotation, the variation in grayscale or color saturation will give rise to the flickering of the telephoto image at a frequency of 12.5 Hz at a speed of 25 rpm. second. The flicker of this frequency can be perceived by the human eye and is therefore unpleasant.

In order to render this effect invisible, consecutive track parts within a track according to a further characteristic feature of the inventive record carrier differ in that they contain regions of a first phase depth and areas of a second phase depth, respectively.

30 In the case of a television program, these track sections contain each information about a television line. If the television picture consists of 625 lines, the switching between one reading system and the other occurs at a frequency of the order of 7.5 kHz. The flicker at such a high frequency is not visible.

In order to ensure correct temporal switching from one detector arrangement to the other during the reading of a registration carrier, in accordance with a further characteristic feature, in addition to an information signal, a pilot signal / marking the transitions between the areas with a first phase depth and the areas may be registered. with a different phase depth and vice versa.

5 This measure may be used where, for example,: only an audio signal is registered on the registration carrier.

If a television signal is detected, the sub-image synchronization pulses or the image synchronization pulses can be used for the switching, and no separate pilot signal need be registered.

An apparatus for reading a recording carrier containing a beam source for producing a reading beam, an objective system for focusing the reading beam into a reading spot on the information layer of the recording carrier, and a radiation sensitive detector system for converting the reading beam that has been modulated by the information structure into a electrical signal is peculiar in that the detector system contains two 20 radiation-sensitive detectors disposed in the information field remote field on either side of a line that effectively crosses the track direction, that the outputs of the detectors are connected to two inputs to a first electronic circuit in which the detector signals are added additively in a first time interval and subtractively in a second time interval, the output of this circuit being connected to the input of a second electronic circuit wherein a switching signal is derived from the signal read from the recording carrier, which switch signal a control input is applied to the first electronic circuit and determines said time intervals.

The fact that the line on whose two sides the detectors are located "effectively across the track direction" means that the projection of this line across the information structure plane is across the track direction.

An apparatus equipped with one servo system intended to hold the reading spot in the center of an information track and containing a radiation-sensitive detector system for generating a position error signal, a control coupling for converting this signal into a control signal for a activator, which can be varied by the radial position of the reading spot, is characterized in that a switchable inverter stage is interposed with a control input connected to the output of the second electronic circuit where the switching signals 10 occur.

This prevents the reading spot from reading a first track, whose areas have a specific phase depth / is directed to a second track, whose areas have a different phase depth.

The invention is explained in more detail below with reference to the schematic drawing, in which fig. 1 is a plan view of part of a first embodiment of a record carrier according to the invention; FIG. 2 shows a tangential section through this registration carrier; FIG. Fig. 3 is a top plan view of a radial section through this record carrier; Fig. 4 is a fragmentary view of another embodiment of a record carrier according to the invention. 5 shows a tangential section through this record carrier; FIG. 6 is a radial section through this record carrier; FIG. 7 shows an embodiment of a reading apparatus according to the invention; FIG. 8 shows the arrangement of the detectors and a first example of the electronic circuit for processing the detector signals; FIG. 9 shows another example of this electronic circuit; FIG. 10 the arrangement of the detectors relative to diffractions of different order.

FIG. 11 shows the variation in the amplitude of the signal read as a function of the phase depth; and FIG. 12 shows the form of a radial error signal in an example of a reading position servo system for radial position 5.

In the figures, corresponding elements are denoted by the same reference numbers.

FIG. 1, 2 and 3 show a first embodiment of a recording carrier according to the invention. FIG. Figure 10 is a top plan view of the record carrier; 2 in tangential section along line II-II of FIG. 1 and FIG. 3 in radial section along line III-III in FIG. 1. The information is contained in several areas 4, e.g. recesses in a substrate 6. These regions are located 15 in tracks 2. Between regions 4 are intermediate regions 5. The tracks 2 are separated from each other by narrow strips 3. The spatial frequency and possibly the lengths of the areas are determined by the information.

The areas in neighboring information traces have different phase depths. As shown in FIG. 3, the grooves in a first groove, a third groove, etc. are deeper than the grooves 4 'in a second groove, a fourth groove, etc. The geometric depth of the grooves 4 and 4' are denoted d1 and d2 · Due to the different depths , 25, the first track, the third track, etc. can be discriminated or optically distinguished from the second track, the fourth track, etc. These tracks can then be placed close to each other.

In a realized embodiment of a record carrier according to the invention, the radial period of the information tracks was 0.85 µm, the width of these tracks 0.5 µm and the width of the strips 0.35 µm.

The information-bearing surface of the record carrier may be reflective e.g. in that a metal layer 7 e.g. aluminum is vacuum-deposited on this surface.

It should be noted that in FIG. 1, 2 and 3, the dimensions of the areas are shown to be greatly increased for the sake of clarity.

f49728 9

FIG. 4 is a top plan view of another embodiment of a record carrier according to the invention. This figure shows a larger part of the registration carrier than fig. 1, so that the individual areas are no longer visible. The information track is now divided into parts a and b, of which parts a contain wells with greater phase depth (areas with deeper recesses) and parts b areas with smaller phase depth.

In FIG. 5, showing an enlarged tangential section through a trace along the line V-V 'in FIG. 4, the depressions with depth d2 are again designated 4 'and the depressions with depth d ^ are designated 4. FIG. 6 is a radial section through this second embodiment along the line VI-VI 'in FIG. 4th

15 In Danish Patent Application No. 4775/73, a method for optical registration of information on a registration carrier is described. A photoresist layer is exposed intermittently according to the information to be inscribed. By subsequent development and possibly etching, the pattern thus produced can be transformed into a depth profile. By selecting a greater radiation intensity during the entry of a first track, a third track, etc. than during the entry of a second track, a fourth track, etc., a record carrier can be obtained whose odd numbered tracks contain recesses having a larger phase depth and the even numbered groove recesses with a smaller phase depth.

FIG. 7 shows an embodiment of an apparatus 30 for reading a record carrier according to the invention. The round plate-shaped record carrier 1 is shown in radial section. Accordingly, the information traces are perpendicular to the drawing plane. It is assumed that the information structure is located on the top of the registration carrier and is reflective, so that reading is performed through the substrate 6. The information structure may be covered by a protective layer 8.

By means of a shaft 16 driven by a motor 15, 10 T49728, the registration carrier can be rotated.

A radiation source 10 e.g. a helium-neon laser or a semiconductor diode laser produces a read beam 11. This beam is reflected by a mirror 125 against an objective system 13 represented schematically by a single lens. In the path of the reading beam is inserted auxiliary lens 14 which ensures that the pupil of the lens system is filled in an optimal way. A reading plaque V of minimal dimensions is then formed on the information structure.

The read beam is reflected by the information structure and as the record carrier rotates, it is modulated according to the order of areas in the track being read. By moving the reading spot and the record carrier relative to each other in a radial direction, the entire information area can be scanned.

The modulated reading beam again passes through the lens system and is reflected by the mirror 12. The beam path contains means for separating the modulated and unmodulated reading beam. These agents may e.g. contain a polarization sensitive part prism and an i - plate, where λ is the wavelength of the reading beam.

For simplicity, it is shown in FIG. 7, said means are constituted by a semi-transparent mirror 17.

25 This mirror reflects the modulated reading beam of a radiation sensitive detector system 20.

The detector system is located in the so-called "remote field for the information structure", ie. in a plane in which the centers of the sub-rays formed by the information structure f in particular the sub-rays of the zero order and the sub-rays of the first order are separated. The detector system may be located in the plane 21, in which an auxiliary lens 18 forms an image of the output pupil of the objective system 13. In FIG. 7, the image 35 C of the point C in the output pupil is shown by dashed lines.

The detector system 20 contains two detectors 22 and 23. These detectors are shown from above in FIG. 8th

In this figure, the direction in which the information track is scanned / indicated by an arrow 34. When the areas of a read track have large phase depth, e.g. ir rad, the output signals from the detectors must be added, while the signals from the detectors must be subtracted if the areas 2ir in the read track have small phase depth, e.g. —Year.

For this purpose, as shown in FIG. 8 can be connected to both an addition coupling 24 and a subtraction coupling 25. The outputs 10 of the couplings 24 and 25 are connected to the two input terminals e and to a coupler 26 having a main terminal e. Depending on a control signal S,

C

provided to the control input of the coupler, this coupler «either conducts the sum signal from detectors 22 and 23 or 15 the difference signal from these detectors to a demodulation coupling 27. In this coupling, the read signal is demodulated and made suitable for reproduction e.g. on a television set 28.

To control coupler 26, a control signal 20 is produced. In addition to the actual information signal, a pilot signal may be registered on the recording carrier, marking the positions of the recording carrier where a transition from areas with a first phase depth to areas with a second phase depth is located. If a television signal with one television image is recorded in each track, the image synchronization pulses or the sub-image synchronization pulses contained in the current television signal can be used to generate the control signal S. These v 30 pulses can always be easily identified.

If information about the lines of a television image is contained in the track parts a and b in accordance with FIG. 4, the line synchronization pulses 31, as shown in FIG. 8, the signal 35 is separated from the demodulation coupling 27 in the line synchronization pulse separator 29. In the coupling 30, e.g. is a bistable multivibrator, the pulses 31 are converted into a control signal Sc to the coupler 26 such that this coupler switches every time a line is read in the television picture.

If each information structure track contains only one type of region, the element 29 is an image-5 synchronization pulse separator and the coupler 26 is changed after reading each track or two television sub-images.

FIG. 9 shows another example of an electronic circuit by which the output signals of detectors 22 10 and 23 can be alternately combined additively and subtractively. The detectors are now connected to a first and second inputs to a difference amplifier 35. The detector 22 is connected directly to this amplifier, while in the connection between the detector 23 and the difference amplifier 35 an inverter 36 and a coupler 37 are inserted, thus that the signal from the detector 23 can be applied to the difference amplifier in inverted or non-inverted form.

In the following, the physical background of the invention will be explained in more detail. The information structure containing neighboring traces with regions and intermediate regions acts as a two-dimensional diffraction grating. The grid divides the reading beam into a zero-order winning beam and a number of first-order sub-beams as well as a number of higher-order sub-beams.

Part of the radiation re-enters the lens system after reflection at the information structure. In the plane of the output pupil of the lens system or in a plane in which an image of this output pupil is formed, the centers of the sub-jets are separated. FIG. 10 shows the situation in plane 21 of FIG. 7th

The circle 40 with center 45 represents the cross-section of the zero order sub-beam in this plane. The circles 41 and 42 with the centers 46 and 47 represent the cross sections of the sub-rays of the orders (+ 1.0) and (- 1.0) displaced in the tangential direction by diffraction. The X-axis and Y-axis in FIG. 2 corresponds to the tangential or tracer direction, and the radial or transverse direction, respectively, of the record carrier. The distance f from the centers 46 and 47 to the Y axis is determined by where p represents the local spatial period of the regions 5 of the track portion to be read and λ the wavelength of the reading beam.

To read the information, the phase variations of the sub-jets of the orders (+ 1.0) and (- 1.0) are used in relation to the zero-order sub-jets.

10 In the shaded areas of FIG. 10, the first-order sub-jets overlap the zero-order jets and interference occurs. The phases of the first order sub jets vary as a result of the movement of the reading spot in the tantential direction relative to the information track. This causes intensity variations in the output pupil or in its image, and these variations can be detected by detectors 22 and 23.

When the center of the reading area coincides with the center of an area, a certain phase difference ψ 20 is produced between the first-order sub-jets and the zero-order sub-jets. This phase difference is called the phase depth of the information structure. Upon the transition from the first area to a second area, the phase of the lower beam of the order (+ 1.0) is increased by 2 π. Therefore, it can be said that during the movement of the reading spot in the tangential direction, the phase of the sub-beam varies with respect to the zero-order sub-beam by ω t. Here ou is a time frequency determined by the spatial frequency of the regions and by the speed at which the reading patch is weighted over the groove.

The phases Φ (+ 1.0) and Φ (- 1.0) of the first-order sub-beams relative to the zero-order sub-beam can be represented by: Φ (+ 1.0) = ψ + ω t 35 Φ (- 1 , 0) = ψ - ω t

The intensity variations due to interference of the first-order sub-beams with the zero-order sub-beam are converted into electrical signals by the detectors 22 and 23. The time-dependent output signals S22 S22 from the detectors 23 and 22 can be represented by: 523 = COS (Ψ + ω 5 S22 = B (ψ) cos (ψ - ω t) X these equations are Β (ψ) a factor proportional to the geometric depth of the depressions It can be assumed that Β (ψ) is zero for ψ = ^.

As shown in FIG. 8, signals S22 10 and S23 are added to each other, giving: 524 = S22 + S23 = 2B W * cos ^ * cos ω Signals S22 and S23 are also subtracted from each other, giving: 525 = S22 ”S23 =” 2B ^ · sin Ψ · sin ω 15 It follows that for a phase depth ψ = ir rad, the amplitude of the signal S24 'is. B (ψ). cos ψ the maximum and the amplitude of the signal S25 'ie. Β (ψ). sin ψ is 3π minimum. For a phase depth ψ = —ζ− row, Β (ψ) .sin ψ is maximum.

FIG. 11 shows the variation of the amplitude A

Β (ψ) cos ψ, of the signal S24, ° 3 the variation of the amplitude A2, ie. Β (ψ) sin ψ, of the signal S25 as a function of the phase depth. For ψ = ^ rad, both A ^ and A_ are zero. A. reaches a maximum for ψ = ir rad. The maximum ^ 3ir 25 for A2 is located at ψ = —j rad. At this phase depth, A still has a significant value. Therefore, in practice z 2ir is chosen the value ψ = —s— for the small phase Λ 2ir depth. The amplitude A2 at the phase depth ψ = —γ- is not substantially smaller than the amplitude A ~ at a phase depth 3ir ^ 30 —7—. However, the amplitude A. changes relatively q 1 3π 2π strongly between the phase depths ψ = —ξ, and ψ = —j.

When the signals from the detectors during the reading are added to each other, the wells with a phase depth of π rad are read optimally. Grooves 2ir 15 149728 with a phase depth of -j row, ie. the recesses in the neighboring grooves can, in fact, be disregarded, so that only small cross talk occurs. Conversely, it is clear that as the signals from the detectors are subtracted from each other during readings, the wells with a phase depth of j rad are read in an optimal manner, while the wells with a phase depth of ir rad can be ignored.

The values for the phase depths of ψ * ir rad and 10 ψ = -j rad specified above need not be strictly adhered to. Deviations of the order of 5% from the large phase depth and of the order of + 15% for the small phase depth are permissible. It can be seen from FIG. 11, that the depth of the phase for the deep grooves is more critical than for the low grooves. The slope for A2 at ψ = π rad is steeper than the slope for A. at 2ir x ψ = =% rad.

In the foregoing, only the first jets are concerned. It is clear that the information structure will also cause higher order diffraction of the radiation. However, the radiant energy of the higher-order diffractions is small, and the diffraction angles are such that only a small portion of the higher-order radiations fall within the pupil of the objective system 13. The influence of the higher-order sub-rays can therefore be neglected.

During reading, the reading spot should be kept exactly centered on the track being read. For this purpose, the reader includes a fine-tuning of the radial position of the reading spot. As shown in FIG. 7, the mirror 12 can be arranged to be rotatable. The axis of rotation 38 of the mirror is perpendicular to the drawing plane so that the reading spot when rotating the mirror 12 is radially displaced. The rotation of the mirror is achieved by a drive element 39. This element can take various forms. It is e.g. an electromagnetic element as shown in FIG. 7 or a piezoelectric element. The drive element is controlled by a control coupling 50 to whose input a radial error signal Sr / ie. a signal that gives an indication of a divergence of the reading position relative to the center of the track.

5 To produce the signal Sr, various methods have been proposed. As can be seen from the Danish patent application 2530/73, two servo spots can be projected onto the information structure in addition to the reading board. These spots are arranged relative to each other such that when the center of the reading plate coincides exactly with the center of the read track, the centers of the servo plates are located at the two edges of the track. There is a separate detector for each servo. The difference between the signals from these detectors is determined by the magnitude and direction of the reading spot radial position error.

In FIG. 12, the fully drawn curve indicates the variation of the signal Sr as a function of the radial position r of the reading spot in the case where only deep traces exist, i. tracks with areas with a large 20 phase depth. When the reading spot is located exactly over a deep track, ie. at the position rQ, 2rQ, etc., the signal Sr is zero. The servo signal for tracking is arranged so that for a negative value of Sr, the mirror 12 of FIG. 7 against the clockwise direction so that the center of the reading spot is placed exactly at the center of the deep groove 2. If Sr has a positive value, the mirror 12 is rotated clockwise. The points D in FIG. 12 are stable points for the servo system.

In addition, on a recording carrier according to the invention, low grooves 2 'are found between the deep grooves 2.

The point E on the curve of Sr corresponding to the center of the groove 2 'is an unstable point. If the reading spot was slightly to the right of the middle of the groove 21, ie. if Sr was positive, mirror 12 would be turned clockwise and the reading spot would be moved further to the right. Similarly, at a deviation to the left of the position of that spot, the reading spot would be moved further to the left. Without further measures, the reading spot would not be able to remain in position on a low track 2 ', but the reading spot would always be located on a deep track.

For reading a low trace or low trace part, the signal Sr according to the invention is inverted before being applied to the control coupling 50. The inverted signal Sr is indicated by the dotted curve in FIG. 12th

The point E on the curve for §s corresponding to the center of the groove 2 'is a stable point and the points D on 10 this curve are unstable points.

In the embodiment of FIG. 7, a combination of an inverter 51 and a coupler 52 is used. As a result, the signal Sr can be applied to the control coupler 50 in inverted or non-inverted form. The coupler 15 52 is controlled by the signal Sc in synchronism with the coupler 26 in FIG. 8. When reading a deep track, the signal Sr is not inverted and when reading a low track it is inverted. When reading a track 2 the part of the curve shown for a strong line is shown with a strong line, and 20 for reading a track 2 'the dashed curve for the section is shown with heavy lines.

A radial error signal can also be generated during the reading by moving the reading spot and the read track periodically relative to each other with small amplitude, e.g. 0.1 times the track width, and with a relatively low frequency of e.g. 30 kHz. The signal emitted by the information detectors then contains an additional component whose frequency and phase are determined by the radial position of the reading spot. The relative motion 30 of the reading spot and groove can be achieved by periodically moving the reading spot in the radial direction. As can be seen from Danish patent application 5376/74, it is also possible to give the information track waveform. A position error signal thus produced must also be inverted when a low trace is read.

Finally, a radial error signal can also be generated by two detectors disposed in the plane 21 on the two sides of a line that is effectively parallel to the track direction as shown in DE-OS 23 42 906.

Subtracting the output signals from the detectors gives a radial error signal Sr · Thus, a radial asymmetry of the radiation distribution in the pupil is determined. Since a deep track, ie. a trace with a phase depth ir for the recesses gives a symmetrical variation over the pupil, this method is only suitable for determining an error signal for the reading spot in relation to a low trace. The signal Sr thus produced varies according to the fully drawn curve of FIG. 12 but with the positions of the deep groove 2 and the low groove 2 'interchanged.

The servo system is now arranged to follow a low 15 track. When following a deep track, the signal Sr must be inverted again. This means that while reading a deep track, a line is actually followed midway between two low tracks.

The detectors for information reading, ie.

20 detectors 22 and 23 of FIG. 10, and the detectors for generating the radial error signal can be combined in the form of four detectors located in the four different quadrants of the X-Y plane. To read the information, the signals from the detectors in the first and the fourth quadrant are added to each other, as are the signals from the detectors in the second and third quadrant. The sum signals thus generated are added to each other or subtracted from each other as indicated above. To generate the radial error signal, the signals from the detectors in the first and second quadrant are first added, as are the signals from the detectors in the third and fourth quadrant. The sum signals thus produced are subtracted so that the signal Sr is obtained.

The invention has been described on the basis of a reflective record carrier. However, it is also possible to apply the invention to a recording carrier with a phase structure read by transmission.

149728 19

If the phase structure contains recesses or elevations, these must be deeper or higher than the recesses and elevations of a reflective record bear.

In addition, the invention may be applied to a tape carrier in the form of a tape. In this case, the term "radial direction" used above must be interpreted as the direction perpendicular to the track direction.

Claims (6)

149728 A record carrier containing information in an optically readable structure comprising trace-spaced regions interspersed, in that said regions have a different impact on a reading beam 5 than the intermediate regions and the strips located between the information tracks, characterized in that by adjacent information tracks differ from each other in that they contain regions of a first phase depth and areas of a second phase depth, respectively. Registration carrier according to claim 1, characterized in that the first phase depth is approximately in rad and the second phase depth approximately in rad. Record carrier according to claim 2, with a reflective information structure, characterized in that the regions of the first phase depth are constituted by depressions having a geometric depth of approximately · and that the regions of the second phase depth are comprised of regions with a geometrical depth of approximately |, where λ is the wavelength of the reading beam. Record carrier according to claim 1 or 3, characterized in that consecutive track parts within a track differ from one another in that they contain areas with a first phase depth and areas with a second phase depth, respectively. Registration carrier according to claims 1, 2, 3 or 4, characterized in that, in addition to an information signal, a pilot signal is registered which marks the transition between the areas with a first phase depth and the areas with a second phase depth and vice versa. Apparatus for reading a recording carrier according to claim 1, comprising a radiation source for producing a reading beam, an objective system for focusing the reading beam into a reading spot on the information layer of the recording carrier and a radiation sensitive detector system for converting it by the information layer.